1. Field of the Invention
The invention relates generally to the field of power supply circuits, and more particularly to a zero-crossover circuit for regulating the output of DC power supply to eliminate turn-on transients of silicon-controlled rectifiers.
2. Description of the Prior Art
It is known in the prior art that regulation of a DC power supply may be obtained by phase control switching of the AC supply. This permits controlling the average power to the lead by changing the on-off duty cycle. Control is accomplished by governing the phase angle of the AC wave at which a silicon-controlled rectifier (SCR) is triggered. SCR regulated DC power supplies are highly efficient and can operate directly from the AC power lines, thus alleviating the need for a power transformer. In addition, these supplies are readily adapted to a "soft-start" capability to limit turn-on surges of current. In phase-controlled SCR regulators, an SCR is triggered when the SCR is forward biased at a point on the input waveform that is dependent on the developed DC voltage. Thus, when more current is drawn from the DC output of the power supply, the output voltage drops, requiring that the control circuit trigger the SCR at an earlier point in the input waveform. This provides more current to raise the output DC voltage. When less current is required from the power supply output, the output voltage increases. The control circuit then triggers the SCR's at a later point on the input waveform, to supply reduced current which lowers the output voltage. Thus, by varying the trigger point at which the SCR devices are switched, it is possible to regulate the DC power supply output voltage.
Referring now to FIG. 1, the operation of a single-phase silicon-controlled regulator is illustrated. Fundamentally, the principle of operation with three or more phases is the same but the more complex waveshapes render it not as useful as an example. FIG. 1(a) shows an AC sine wave as would be applied to the input of the regulated power supply. In the prior art phase-controlled SCR regulator, each SCR is forward biased. Once turned on, the SCR will continue to conduct until the forward voltage again goes to zero. For example, during the positive one-half cycle of the AC sine wave voltage, SCR Q1 is biased to be triggered on at the peak of the sine wave as shown by the solid vertical line 130 of FIG. 1(b). When the AC waveform returns to the zero level, as at point 132, the SCR stops conduction. When the AC sine wave reaches the peak of the negative half of the sine wave, then SCR Q2 is triggered on as shown at line 134. When more current is demanded from the DC output of the power supply, the output voltage across C1 will drop. This causes the prior art SCR regulator circuit to trigger Q1 and Q2 earlier, as indicated by lines 136 and 138, and results in providing more current to raise the output voltage to the desired regulated value. If the load demands less current from this DC output, the output voltage will rise. The control circuit thereupon triggers the silicon control rectifiers Q1 and Q2 at a later point on the waveform as indicated by lines 140 and 142. This results in less average current being provided to the load and a lower output voltage results. Thus, by varying the trigger point at which the silicon-controlled rectifiers all switched therefor, it is possible to regulate the DC output voltage.
It has been found that an SCR when triggered will change state from turn-off to turn-on within one microsecond or less. This substantially instantaneous switching at high voltage and high current may generate high intensity electromagnetic transient voltages. These voltages may be coupled inductively or conductively to cause severe electromagnetic interference to other equipment connected to the common AC power lines located in proximity to the phase-control SCR regulator circuit.
It is desired therefore to provide a regulator circuit which maintains the high efficiency of the phase-controlled SCR regulated DC power supply while minimizing the generation of electromagnetic interference transients. The present invention eliminates these transients by triggering the silicon-controlled rectifiers only when the AC sine wave is crossing through the zero point.